Internal combustion engines having variable camshaft phasing are well known. Camshaft phasers are used to improve fuel economy and reduce formation of nitrogen oxides (NOx) by adjusting the timing of Intake Valve Opening (IVO), Exhaust Valve Closing (EVC), or both IVO and EVC independently so that the engine cylinders are held at their dilution limit during operating conditions of partial load. Cylinder dilution is defined as the fraction of burned gas left over from the previous combustion event that is contained in the cylinder during the compression stroke.
Under steady state operating conditions, the optimum positions of the cam phaser or cam phasers as a function of engine speed and load is readily determined by mapping of engine performance on an engine dynamometer. The test results are tabulated in look-up tables which are programmed into an Engine Control Module (ECM) that governs the position of the cam phasers for any given condition of engine speed and load. This procedure yields appropriate dilution control for steady-state engine operation.
A problem arises, however, during transient periods between different engine states of speed and load requiring movement of each phaser from a first position to a second position, during which time it is necessary to carefully coordinate phaser positions to the load response of the engine. The load response rate of an engine following a driver input to the accelerator pedal depends on several engine design features, and cam phaser response depends upon both its design as well as the particular engine operating condition. Depending on the response rate of the cam phasers relative to the load response of the engine, periods of either excessive dilution or sub-optimal dilution can result. If phaser response is relatively fast, the result will be too much dilution when engine load is increasing, and too little dilution when load is decreasing. If phaser response is relatively slow, the opposite occurs. Excessive dilution causes unstable combustion, while sub-optimal dilution reduces fuel economy and increases NOx emissions.
In prior art practice, to avoid excessive dilution and unstable combustion, the phaser position is calibrated conservatively, resulting in sub-optimum fuel economy and increased NOx under real vehicle driving conditions.
What is needed in the art is a method for maintaining desired optimum dilution levels during transient engine operation.
It is a principal object of the present invention permit optimum fuel mixture dilution to be maintained during transient engine operation, resulting in improved overall fuel economy and NOx emission control.